Method of forming semiconductor device

ABSTRACT

A method includes encapsulating a device die in an encapsulating material, forming a first dielectric layer over the device die and the encapsulating material, forming first redistribution lines extending into the first dielectric layer to electrically couple to the device die, forming an alignment mark over the first dielectric layer, wherein the alignment mark includes a plurality of elongated strips, forming a second dielectric layer over the first redistribution lines and the alignment mark, and forming second redistribution lines extending into the second dielectric layer to electrically couple to the first redistribution lines. The second redistribution lines are formed using the alignment mark for alignment.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/834,103, entitled “Method of Forming Semiconductor Device,” filed onMar. 30, 2020, which is a continuation of U.S. patent application Ser.No. 15/966,512, entitled “Method of Forming Semiconductor Device,” filedon Apr. 30, 2018, now U.S. Pat. No. 10,607,941, issued Mar. 31, 2020,which applications are incorporated herein by reference

BACKGROUND

With the evolving of semiconductor technologies, semiconductorchips/dies are becoming increasingly smaller. In the meantime, morefunctions need to be integrated into the semiconductor dies.Accordingly, the semiconductor dies need to have increasingly greaternumbers of I/O pads packed into smaller areas, and the density of theI/O pads rises quickly over time. As a result, the packaging of thesemiconductor dies becomes more difficult, which adversely affects theyield of the packaging.

Conventional package technologies can be divided into two categories. Inthe first category, dies on a wafer are packaged before they are sawed.This packaging technology has some advantageous features, such as agreater throughput and a lower cost. Further, less underfill or moldingcompound is needed. However, this packaging technology also suffers fromdrawbacks. Since the sizes of the dies are becoming increasinglysmaller, and the respective packages can only be fan-in type packages,in which the I/O pads of each die are limited to a region directly overthe surface of the respective die. With the limited areas of the dies,the number of the I/O pads is limited due to the limitation of the pitchof the I/O pads. If the pitch of the pads is to be decreased, solderbridges may occur. Additionally, under the fixed ball-size requirement,solder balls must have a certain size, which in turn limits the numberof solder balls that can be packed on the surface of a die.

In the other category of packaging, dies are sawed from wafers beforethey are packaged. An advantageous feature of this packaging technologyis the possibility of forming fan-out packages, which means the I/O padson a die can be redistributed to a greater area than the die, and hencethe number of I/O pads packed on the surfaces of the dies can beincreased. Another advantageous feature of this packaging technology isthat “known-good-dies” are packaged, and defective dies are discarded,and hence cost and effort are not wasted on the defective dies.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 18 illustrate intermediate stages in the formation of apackage in accordance with some embodiments.

FIGS. 19 and 20 illustrate intermediate stages in the formation of apackage in accordance with some embodiments.

FIGS. 21 through 27 illustrate the cross-sectional views of intermediatestages in the formation of a package in accordance with someembodiments.

FIGS. 28A, 28B, 28C, and 28D illustrate the top views of some alignmentmarks in accordance with some embodiments.

FIGS. 29A-1 and 29A-2 illustrate an alignment mark and correspondingbrightness-contrast signal intensity, respectively in accordance withsome embodiments.

FIGS. 29B-1 and 29B-2 illustrate an alignment mark and the correspondingbrightness-contrast signal intensity, respectively in accordance withsome embodiments.

FIGS. 29C-1 and 29C-2 illustrate an alignment mark with inversed patternand the corresponding brightness-contrast signal intensity, respectivelyin accordance with some embodiments.

FIG. 30 illustrates a process flow for forming a package in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

An Integrated Fan-Out (InFO) package including alignment marks havinggrating patterns and the method of forming the same are provided inaccordance with various embodiments. The intermediate stages of formingthe InFO package are illustrated in accordance with some embodiments.Some variations of some embodiments are discussed. Throughout thevarious views and illustrative embodiments, like reference numbers areused to designate like elements.

FIGS. 1 through 18 illustrate the cross-sectional views of intermediatestages in the formation of a package in accordance with someembodiments. The steps shown in FIG. 1 through 18 are also illustratedschematically in the process flow 300 shown in FIG. 30 .

Referring to FIG. 1 , carrier 20 is provided, and release film 22 iscoated on carrier 20. Carrier 20 is formed of a transparent material,and may be a glass carrier, a ceramic carrier, an organic carrier, orthe like. Carrier 20 may have a round top-view shape, and may have asize of a silicon wafer. Release film 22 is in physical contact with thetop surface of carrier 20. Release film 22 may be formed of aLight-To-Heat-Conversion (LTHC) coating material. Release film 22 may beapplied onto carrier 20 through coating. In accordance with someembodiments of the present disclosure, the LTHC coating material iscapable of being decomposed under the heat of light/radiation (such as alaser beam), and hence can release carrier 20 from the structure formedthereon.

In accordance with some embodiments, as also shown in FIG. 1 , polymerbuffer layer 24 is formed on LTHC coating material 22. Polymer bufferlayer 24 may be formed of polybenzoxazole (PBO), polyimide,benzocyclobutene (BCB), or another applicable polymer.

FIGS. 2 through 4 illustrate the formation of metal posts 32. Therespective process is illustrated as process 302 in the process flowshown in FIG. 30 . Throughout the description, metal posts 32 arealternatively referred to as through-vias 32 since metal posts 32penetrate through the subsequently dispensed encapsulating material.

Referring to FIG. 2 , metal seed layer 25 is formed, for example,through Physical Vapor Deposition (PVD). Metal seed layer 25 may be inphysical contact with polymer buffer layer 24. In accordance with someembodiments of the present disclosure, metal seed layer 25 includes atitanium layer and a copper layer over the titanium layer. In accordancewith alternative embodiments of the present disclosure, metal seed layer25 includes a copper layer contacting buffer dielectric layer 24.

As also shown in FIG. 2 , photo resist 26 is formed over metal seedlayer 25. A light-exposure is then performed on photo resist 26 using aphoto lithography mask (not shown). After a subsequent development,openings 28 are formed in photo resist 26. Some portions of metal seedlayer 25 are exposed through openings 28.

Next, as shown in FIG. 3 , metal posts 32 are formed by plating ametallic material in openings 28. The plated metallic material may becopper or a copper alloy. The top surfaces of metal posts 32 are lowerthan the top surface of photo resist 26, so that metal posts 32 areconfined by openings 28. Metal posts 32 may have substantially verticaland straight edges. Alternatively, metal posts 32 may have a sand-timershape in a cross-sectional view, with the middle parts of metal posts 32being narrower than the respective top parts and bottom parts.

In subsequent steps, photo resist 26 is removed, and the underlyingportions of metal seed layer 25 are exposed. The exposed portions ofmetal seed layer 25 are then removed in an etching step, for example, ina plurality of anisotropic and/or isotropic etching steps. The edges ofthe remaining seed layer 25 are thus substantially co-terminus with therespective overlying portions of metal posts 32. The resulting metalposts 32 are illustrated in FIG. 4 . Throughout the description, theremaining portions of metal seed layer 25 are considered as parts ofmetal posts 32, and are not illustrated separately. The top-view shapesof metal posts 32 include, and are not limited to, circular shapes,rectangles, hexagons, octagons, and the like. After the formation ofmetal posts 32, polymer buffer layer 24 is exposed.

FIG. 5 illustrates the placement/attachment of device die 36. Therespective process is illustrated as process 304 in the process flowshown in FIG. 30 . Device die 36 is attached to polymer buffer layer 24through Die-Attach Film (DAF) 38, which is an adhesive film. DAF 38 maybe pre-attached on device die 36 before device die 36 is placed onpolymer buffer layer 24. Accordingly, DAF 38 and device die 36, beforeattached to polymer buffer layer 24, are in combination an integratedpiece. Device die 36 may include a semiconductor substrate having a backsurface (the surface facing down) in physical contact with DAF 38.Device die 36 may include integrated circuit devices (such as activedevices, which include transistors, for example, not shown) at the frontsurface (the surface facing up) of the semiconductor substrate. Inaccordance with some embodiments of the present disclosure, device die36 is a logic die, which may be a Central Processing Unit (CPU) die, aGraphic Processing Unit (GPU) die, a mobile application die, a MicroControl Unit (MCU) die, an input-output (IO) die, a BaseBand (BB) die,or an Application processor (AP) die. Since carrier 20 is at waferlevel, although one device die 36 is illustrated, a plurality ofidentical device dies 36 is placed over polymer buffer layer 24, and maybe allocated as an array including a plurality of rows and a pluralityof columns.

In accordance with some embodiments, metal pillars 42 (such as copperpillars) are pre-formed as portions of device die 36, and metal pillars42 are electrically coupled to the integrated circuit devices such astransistors (not shown) in device die 36. In accordance with someembodiments of the present disclosure, a dielectric material such as apolymer fills the gaps between neighboring metal pillars 42 to form topdielectric layer 44. Top dielectric layer 44 may also include a portioncovering and protecting metal pillars 42. Polymer layer 44 may be formedof PBO or polyimide in accordance with some embodiments of the presentdisclosure.

Next, device die 36 and metal posts 32 are encapsulated in encapsulatingmaterial 48, as shown in FIG. 6 . The respective process is illustratedas process 306 in the process flow shown in FIG. 30 . Encapsulatingmaterial 48 fills the gaps between neighboring metal posts 32 and thegaps between metal posts 32 and device die 36. Encapsulating material 48may include a molding compound, a molding underfill, an epoxy, and/or aresin. The top surface of encapsulating material 48 is higher than thetop ends of metal pillars 42. When formed of molding compound,encapsulating material 48 may include a base material, which may be apolymer, a resin, an epoxy, or the like, and filler particles in thebase material. The filler particles may be dielectric particles of SiO₂,Al₂O₃, silica, or the like, and may have spherical shapes. Also, thespherical filler particles may have a plurality of different diameters.Both the filler particles and the base material in encapsulatingmaterial 48 may be in physical contact with polymer buffer layer 24.

In a subsequent step, a planarization process such as a ChemicalMechanical Polish (CMP) process or a mechanical grinding process isperformed to thin encapsulating material 48 and dielectric layer 44,until metal posts 32 and metal pillars 42 are all exposed. Therespective process is also illustrated as process 306 in the processflow shown in FIG. 30 . Due to the planarization process, the top endsof metal posts 32 are substantially level (coplanar) with the topsurfaces of metal pillars 42, and are substantially coplanar with thetop surface of encapsulating material 48. Metal posts 32 arealternatively referred to as through-vias 32 in subsequent paragraphssince they penetrate through encapsulating material 48.

FIGS. 7 through 15 illustrate the formation of a front-sideredistribution structure. FIGS. 7 through 10 illustrate the formation ofa first layer of Redistribution Lines (RDLs), an alignment mark, and therespective dielectric layer. Referring to FIG. 7 , dielectric layer 50is formed. The respective process is illustrated as process 308 in theprocess flow shown in FIG. 30 . In accordance with some embodiments ofthe present disclosure, dielectric layer 50 is formed of a polymer suchas PBO, polyimide, or the like. The formation method includes coatingdielectric layer 50 in a flowable form, and then curing dielectric layer50. In accordance with alternative embodiments of the presentdisclosure, dielectric layer 50 is formed of an inorganic dielectricmaterial such as silicon nitride, silicon oxide, or the like. Theformation method may include coating, Chemical Vapor Deposition (CVD),Atomic Layer Deposition (ALD), Plasma-Enhanced Chemical Vapor Deposition(PECVD), or other applicable deposition methods. Via openings 52 arethen formed. The respective process is also illustrated as process 308in the process flow shown in FIG. 30 . In accordance with someembodiments in which dielectric layer 50 is formed of a photo sensitivematerial such as PBO or polyimide, the formation of openings 52 involvesa photo exposure using a lithography mask (not shown), and a developmentstep. Through-vias 32 and metal pillars 42 are exposed through viaopenings 52.

Next, referring to FIG. 8 , metal seed layer 54 is deposited. Inaccordance with some embodiments of the present disclosure, metal seedlayer 54 includes a titanium layer and a copper layer over the titaniumlayer. The formation method may include, for example, PVD. Metal seedlayer 54 extends into openings 52, and contacts through-vias 32 andmetal pillars 42.

FIG. 9 illustrates the formation and the patterning of photo resist 56.Metal seed layer 54 has some portions exposed to the openings in photoresist 56. A plating process is then performed to form metal regions 58.In accordance with some embodiments of the present disclosure, metalregions 58 comprise copper or a copper alloy. The plating may includeelectro-chemical plating or electro-less plating.

In a subsequent process, photo resist 56 is removed, and the underlyingportions of metal seed layer 54 are exposed. One or a plurality ofetching processes are then performed to remove the exposed metal seedlayer 54. In accordance with some embodiments, a first etching processis performed to etch the copper layer in metal seed layer 54, followedby a second etching process to etch the titanium layer in metal seedlayer 54. As a result, RDLs 60 and alignment mark 62 are formed, and theresulting structure is shown in FIG. 10 . The respective process isillustrated as process 310 in the process flow shown in FIG. 30 . Eachof RDLs 60 and alignment mark 62 includes a remaining portion of metalseed layer 54 and a portion of the plated metal regions 58.

RDLs 60 include vias 60A formed in dielectric layer 50 to connect tometal pillars 42 or through-vias 32, and metal traces (metal lines) 60Bover dielectric layer 50. Although not shown, the top surfaces of theportions of metal traces 60B grown from openings 52 (FIG. 8 ) may berecessed lower than the top surfaces of the portions of metal traces 60Bdirectly overlying dielectric layer 50.

Alignment mark 62 includes a plurality of grating strips 64, which incombination form the alignment mark. Grating strips 64 also define thecontour of alignment mark 62, and the shape of the contour may be usedto identify alignment mark 62. The plurality of grating strips 64 inalignment mark 62 are electrically floating. Furthermore, each of strips64 of alignment mark 62 may be fully isolated from other conductivefeatures except other parts of alignment mark 62. Alternatively stated,the top surfaces, bottom surfaces, and the sidewalls of all gratingstrips 64 are in contact with either a dielectric material or anotherpart of alignment mark 62. FIGS. 28A, 28B, 28C, and 28D illustrate thetop views of some examples of alignment marks 62, each including aplurality of elongated metal strips 64. Elongated metal strips 64 may beparallel to each other, with each having a uniform width. Elongatedmetal strips 64 may have a uniform pitch, with pitches P1 and P2 equalto each other, for example. In accordance with some embodiments of thepresent disclosure, some of elongated metal strips 64 may have pitchesdifferent from the pitches of other metal strips 64, for example, withpitches P1 and P2 different from each other.

In accordance with some embodiments of the present disclosure, the widthW1 (and W2) of alignment mark 62 are small, and may be close to theminimum allowed (or can be formed) width of the corresponding RDL. Forexample, width W1 and/or W2 may be between the minimum width and about125 percent the minimum width. The minimum width is the width of metalstrips 64 that is either the smallest width of the corresponding RDLthat can be formed using the corresponding technology without incurringreliability problem such as the delamination between metal strips 64 andthe underlying dielectric layer 50 (FIG. 11 ), and/or delaminationbetween metal strips 64 and the overlying dielectric layer 66 (FIG. 17). It is appreciated that the minimum width is related to the process,the production tool, and the material of metal strips 64 and theneighboring dielectric layers. For example, the minimum width of thecorresponding RDL may be due to process factors such as the limitationof photo resist or the lithography process. In accordance with someembodiments, when the widths of metal strips are smaller about 4 μm,delamination was observed. Accordingly, the widths W3 and W4 are greaterthan 4 μm. In accordance with some embodiments, widths W3 (and W4) is inthe range between about 5 μm and about 10 μm. In accordance with someembodiments, at least some of metal strips 64 have a length-to-widthratio greater than about 5. The length-to-width ratio may also begreater than about 10 or greater, depending on the size of alignmentmark 62, and the larger the alignment mark 62, the greater thelength-to-width ratio can be. Making strips 64 to be elongated allowsthe widths of strips 64 to be minimized without the concern ofdelamination. Otherwise, if both the length and the width of thefeatures 64 in alignment mark 62 are close to each other, features 64need to be made larger in order to be free from delamination. Inaccordance with some embodiments of the present disclosure, lengths L1and L2 of alignment mark 62 may in the range between 30 μm and about 120μm, and width W1 and W2 of alignment mark 62 may in the range between 20μm and about 120 μm.

The alignment mark 62 as shown in FIG. 28A has a contour with asignature pattern, which has the shape of the letter “L” in theillustrated example. The signature pattern of alignment mark 62 may haveother shapes including, and not limited to, crosses, rectangles,squares, or the like. The signature pattern of alignment mark 62 mayalso have the shapes of other letters such as letter “H,” Letter “A,”Letter “C,” etc. The signature pattern, which is letter “L” in FIG. 28A,is defined by the outer contour of metal strips 64. The grating strips64 themselves, how, do not form the lines in the letters directly. Forexample, if letter “H” is the signature pattern, since H includes twovertical line and a horizontal line connecting the two vertical lines,each of the two vertical line and the horizontal line may include aplurality of discrete grating strips.

FIG. 28B illustrates an example of alignment mark 62, which alsoincludes a plurality of metal strips 64. The signature pattern, which isletter “L” in the example, in FIG. 28B, is defined by a void in metalstrips 64, with metal strips 64 not extending into the void.Alternatively stated, the alignment mark as shown in FIG. 28B has aninversed pattern since the signature pattern “L” is defined by the voidin metal strips 64, rather than metal strips 64.

FIG. 28C illustrates alignment mark 62 in which metal strips 64 areconnected to form a ring. Alternatively stated, the signature pattern inFIGS. 28C (and 28D) is a hollow pattern with grating strips lining thecontour. In the embodiments as illustrated, the ring is fully closed. Inaccordance with other embodiment, the ring may be partially closed, forexample, with one or two of the illustrated metal strips 64 in theillustrated ring not formed. FIG. 28D illustrates alignment mark 62 inwhich metal strips 64 are connected to form two rings, with the outerring enclosing the inner ring. In accordance with some embodiments ofthe present disclosure, there may be a plurality of rings (with eachbeing similar to what is shown in FIG. 28C) placed side by side, and theplurality of rings in combination form alignment mark 62. The pluralityof rings may be placed with any directions, positions relative to eachother.

FIGS. 29A-1, 29A-2, 29B-1, 29B-2, 29C-1, and 29C-2 illustrate someexamples of alignment marks and the corresponding signals measured fromthe alignment marks. FIGS. 29A-1, 29B-1, and 29C-1 are alignment marks.FIG. 29A-2 is the brightness-contrast signal intensity obtained from thealignment mark shown in FIG. 29A-1 . FIG. 29B-2 is thebrightness-contrast signal intensity obtained from the alignment markshown in FIG. 29B-1 . FIG. 29C-2 is the brightness-contrast signalintensity obtained from the alignment mark shown in FIG. 29C-1 . Thebrightness-contrast signal intensity values represent the contrast ofbrightness when the alignment marks are scanned from left to right atthe positions of lines 110 in FIGS. 29A-1, 29B-1 , and 29C-1.Accordingly, the highest peaks of the signal intensity are at the edgesof alignment marks.

Referring to FIG. 29C-1 , which illustrates a bulk alignment mark, thereare two edges crossed by line 110, with one on the left and one on theright. The signal intensity values of the two edges are reflected inFIG. 29C-2 as peaks 114. FIG. 29C-1 also illustrates a plurality ofgrains 112 in alignment marks. Grains 112 may be the grains of copper,for example. The grains 112 and the surrounding parts of the respectivealignment mark have difference in brightness, which results in thecontrast in brightness, and hence peaks 116 in FIG. 29C-2 are generated.Peaks 116 are lower than peaks 114. Peaks 114 are used to determinewhere the boundaries of alignment marks are, and peaks 116 act as thenoise that adversely affect the determination of the boundaries (hencethe image) of the alignment mark. In the manufacturing process, theremay be dielectric layers (such as layers 66, 72, and 76 in FIG. 17 )covering alignment marks, causing the image of alignment marks to beblurred. The different between the heights of peaks 114 and peaks 116thus will be reduced. Furthermore, the alignment marks may be damaged incertain process steps, such as the steps shown in FIG. 22 . This alsocauses the reduction of the different between the heights of peaks 114and 116.

Referring to FIG. 29A-1 , metal strips 64 in accordance with someembodiments of the present disclosure have small widths, which means thegrains in metal strips 64 will be limited inside the narrow metalstrips. The roughness of the metal strips 64 is thus reduced, and thepeaks resulting from the grain boundaries (rather than the edges ofmetal strips 64) are at least lowered, and possibly eliminated. FIG.29A-2 schematically illustrates a part of the signal intensity measuredfrom the alignment mark 62 shown in FIG. 29A-1 . It is observed that nopeaks are generated from grain boundaries, and hence the signalsresulted from the edges of metal strips are more significant, and areeasier to be distinguished. This results in the improvement in theimaging of alignment marks since the edges of alignment marks areidentified through the brightness-contrast signals.

FIG. 29B-2 schematically illustrates a part of the brightness-contrastsignal intensity measured from the alignment mark shown in FIG. 29B-1 ,which has an inversed pattern. It is observed that the peaks resultedfrom the grain boundaries are also not present, and hence the signalsresulted from the edges of metal strips are more significant, and areeasier to be distinguished.

Comparing the signals shown in FIGS. 29A-2, 29B-2, and 29C-2 , it isfound that even if the brightness-contrast signal is less clear thanwhat are shown due to the more overlying dielectric layers and/or thedamage of the alignment marks, the brightness-contrast signals shown inFIGS. 29A-2 and 29B-2 are still much easier to be used to determine thepositions of the alignment marks than the signals shown in FIG. 29C-2 .Accordingly, the alignment marks with grating patterns are improved overthe bulk alignment marks such as what is shown in FIG. 29C-1 .

For the easy identification of the patterns, the alignment marks 62 maynot be a grid pattern that includes a first plurality of parallel stripsand a second plurality of parallel strips crossing, and perpendicularto, the first plurality of parallel strips. The grid pattern makes thedistinguishing of the alignment marks to be more difficult.

Referring back to FIG. 11 , dielectric layer 66 is formed over thestructure shown in FIG. 10 . The respective process is illustrated asprocess 312 in the process flow shown in FIG. 30 . Via openings 68 arethen formed in dielectric layer 66. The respective process isillustrated as process 314 in the process flow shown in FIG. 30 .Dielectric layer 66 covers alignment mark 62 and RDLs 60. Some portionsof RDLs 60 are exposed through the via openings. Dielectric layer 66 maybe formed using a material selected from the same group of candidatematerials for forming dielectric layer 50, which may include PBO,polyimide, BCB, or other organic or inorganic materials.

Referring to FIG. 12 , RDLs 70 are formed. The respective process isalso illustrated as process 314 in the process flow shown in FIG. 30 .The formation process of RDLs 70 may be essentially the same as theformation of RDLs 60. RDLs 70 also include via portions extending intothe via openings in dielectric layer 66 to contact RDLs 60, and metaltrace portions directly over dielectric layer 66. The formation of RDLs70 may include forming a metal seed layer, forming a patterned mask(such as a photo resist), plating RDLs 70, and then removing thepatterned mask and undesirable portions of the seed layer.

In the formation of openings 68 (FIG. 11 ) and the formation of RDLs 70(FIG. 12 ), alignment mark 62 is used to align the positions of openings68 and the trace portions of RDLs 70 to the desirable positions. In thealignment, alignment mark 62 is first found, and then the positions ofopenings 68 and RDLs 70 are determined based on the position ofalignment mark 62. It is appreciated that there may be multiplealignment marks in the package, with each on a side of the respectivedie. Alignment mark 62 is viewed from top through transparent (or atleast partially transparent) dielectric layer 66. By adopting thegrating patterns, alignment mark 62 can be clearly visible, and theaccuracy in the alignment is improved.

FIG. 13 illustrates the formation of dielectric layer 72 over dielectriclayer 66 and RDLs 70. The respective process is illustrated as process316 in the process flow shown in FIG. 30 . Next, via openings 73 areformed in dielectric layer 72. The respective process is illustrated asprocess 318 in the process flow shown in FIG. 30 . Dielectric layer 72may be formed of a material selected from the same group of candidatematerials for forming dielectric layers 50 and 66.

Referring to FIG. 14 , RDLs 74 are formed. The respective process isalso illustrated as process 318 in the process flow shown in FIG. 30 .The formation process of RDLs 74 may be essentially the same as theformation of RDLs 60. RDLs 74 may be formed of a metal or a metal alloyincluding aluminum, copper, tungsten, or alloys thereof. It isappreciated that although in the illustrated examples of embodiments,three layers of RDLs (60, 70 and 74) are formed, the package may haveother number of RDL layers such as one layer, two layers, or more thanthree layers.

In the formation of RDLs 74, alignment mark 62 is used to align thepositions of RDLs 74 (and the positions of the corresponding viaportions) to RDLs 70. Alignment mark 62 is viewed from top throughtransparent (or at least partially transparent) dielectric layers 72 and66. By adopting the grating patterns, alignment mark 62 can be clearlyvisible (with a high clarity score judged by the production tool usedfor alignment) through dielectric layers 72 and 66, and the accuracy inthe alignment is improved. As a comparison, if the bulk pattern such aswhat is shown in FIG. 29C-1 is adopted, the determination of thealignment mark is more likely to fail.

FIG. 15 illustrates the formation of dielectric layer 76. Dielectriclayer 76 may be formed of a material selected from the same group ofcandidate materials for forming dielectric layers 50, 66, and 72. Forexample, dielectric layer 76 may be formed using PBO, polyimide, or BCB.Openings 77 are formed in dielectric layer 76 to expose the underlyingmetal pads, which are parts of RDLs 74. The positions of openings 77 mayalso be determined using alignment mark 62 for alignment, which isvisible through dielectric layers 76, 72, and 66.

FIG. 16 illustrates the formation of Under-Bump Metallurgies (UBMs) 78in accordance with some embodiments. The respective process isillustrated as process 320 in the process flow shown in FIG. 30 . Inaccordance with some embodiment of the present disclosure, UBMs 78 areformed to extend into the openings in dielectric layer 76 in order tocontact the metal pads in RDLs 74. UBMs 78 may be formed of nickel,copper, titanium, or multi-layers thereof. In accordance with someembodiments, UBMs 78 include a titanium layer and a copper layer overthe titanium layer.

Electrical connectors 80 are then formed in accordance with someembodiments. The formation of electrical connectors 80 may includeplacing solder balls on the exposed portions of UBMs 78, and thenreflowing the solder balls. In accordance with alternative embodimentsof the present disclosure, the formation of electrical connectors 80includes performing a plating step to form solder layers over UBMs 78,and then reflowing the solder layers. Electrical connectors 80 may alsoinclude non-solder metal pillars, or metal pillars and solder caps overthe non-solder metal pillars, which may also be formed through plating.Throughout the description, the structure including dielectric bufferlayer 24 and the overlying structure in combination is referred to aspackage 100, which is a composite wafer (and also referred to ascomposite wafer 100 hereinafter) including a plurality of device dies36.

Next, composite wafer 100 is demounted from carrier 20, for example, byprojecting a laser beam on release film 22. Release film 22 isdecomposed under the heat of the laser beam. The resulting compositewafer 100 is illustrated in FIG. 17 . Next, openings 82 are formed indielectric buffer layer 24, for example, through laser drilling. Whenthrough-vias 32 includes titanium layers at their bottoms, and thetitanium layers come from the metal seed layer 25 (FIG. 2 ), thetitanium layer may be removed through etching, hence exposing the copperin through-vias 32.

Composite wafer 100 may then be singulated in a die-saw step. Therespective process is illustrated as process 322 in the process flowshown in FIG. 30 . For example, a blade may saw-through the scribe lines84 to separate wafer 100 into a plurality of identical packages 86, eachhaving the structure as illustrated in accordance with some examples.The die-saw may pass through some or all of alignment mark 62. As aresult, the a resulting packages 86 may include a part of alignment mark62, or does not include any part of alignment mark 62 since alignmentmark 62 has been cut in the die-saw. For example, package 86 may includean entirety of one or more of the metal strips 64 (FIGS. 28A through28D), and/or may include a part of one or more of metal strips 64,depending on the sawing. For example, in FIGS. 28A through 28C, the leftparts of alignment marks 62 may be sawed, while the right parts ofalignment marks 62 are left in the final package 86, or vice versa. Itis also possibly that the upper parts of alignment marks 62 are sawed,while the lower part of alignment marks 62 are left in the final package86, or vice versa. The sawed percentage of alignment marks 62 may be anypercentage of the illustrated portions.

FIG. 18 illustrates the bonding of package 86 to package component 88through solder regions 80. In accordance with some embodiments of thepresent disclosure, package component 88 is a package substrate, whichmay be a coreless substrate or a substrate having a core. In accordancewith other embodiments of the present disclosure, package component 88is a printed circuit board or a package. Underfill 90 may be dispensedbetween package 86 and package component 88. Package 86 may also bebonded to package 200 through solder regions 206. In accordance withsome embodiments, package 200 includes device dies 202 and substrate204. Dies 202 may be memory dies such as Dynamic Random Access Memory(DRAM) dies. Underfill 208 may be disposed between package 86 andpackage 200. The resulting package in FIG. 18 is referred to as package220.

FIGS. 19 through 27 illustrate the cross-sectional views of intermediatestages in the formation of packages in accordance with some embodimentsof the present disclosure. Unless specified otherwise, the materials andthe formation methods of the components in these embodiments areessentially the same as the like components, which are denoted by likereference numerals in the embodiments shown in FIGS. 1 through 18 . Thedetails regarding the formation process and the materials of thecomponents shown in FIGS. 19 through 27 may thus be found in thediscussion of the embodiments shown in FIGS. 1 through 18 .

FIGS. 19 and 20 illustrate some embodiments. These embodiments aresimilar to the embodiments in FIGS. 1 through 18 , except that theportions of dielectric layers 72 and 76 are opened for a clearer view ofalignment mark. The resulting package 86 is shown in FIG. 19 . Theformation process of package 86 is similar to what are shown in FIGS. 1through 18 , except opening 94 is formed, and opening 94 penetratesthrough dielectric layers 72 and 76. Opening 94 includes two portions,with the first portion in dielectric layer 72, and the second portion indielectric layer 76. The first portion of opening 94 is formed in thesame process for forming the openings 73 in FIG. 13 . The lithographymask is modified so that when openings 73 are formed, the lower portionof opening 94 is formed. The second portion of opening 94 is formed inthe same process for forming the openings 77 in FIG. 15 . Thelithography mask is modified so that when openings 77 are formed, theupper portion of opening 94 is formed. Since dielectric layer 76 will befilled into opening 94, when the upper portion of opening 94 is formed,the portion of dielectric layer 76 filling the lower portion of opening94 is also removed.

FIG. 20 illustrates the package 220 including package 86, and packages88 and 200 bonded to package 86. Underfill 90 extends into the remainingport of opening 94 (if any is left).

FIGS. 21 through 27 illustrate the intermediate stages in the formationof packages in accordance with some embodiments. These embodiments aresimilar to the embodiments in FIGS. 1 through 18 , except that theportions of dielectric layers 66, 72 and 76 directly over alignment mark62 are all opened for a clearer view of alignment mark 62. Accordingly,when alignment mark 62 is used for aligning the formation of RDLs 70,74, and UBMs 78, alignment mark 62 is exposed.

The initial processes in accordance with these embodiments are similarto what are shown in FIG. 10 . Next, referring to FIG. 21 , dielectric66 is formed, followed by the formation of openings 68 and 94. Alignmentmark 62 is exposed to opening 94. Also, the top surface of dielectriclayer 50 is also exposed to opening 94.

FIG. 22 illustrates the formation of metal seed layer 70A, which mayinclude a titanium layer and a copper layer over the titanium layer inaccordance with some embodiments of the present disclosure. Metal seedlayer 70A extends into openings 68 and 94. Next, referring to FIG. 23 ,photo resist 124 is formed and then patterned. An entirety of opening 94is filled by photo resist 124.

Next, metal regions 70B are plated into the openings in photo resist124, followed by the removal of photo resist 124. Some portions of metalseed layer 70A are then exposed. The exposed portions of metal seedlayer 70A are then etched, revealing the underlying alignment mark 62.RDLs 70 are also formed, and include metal seed layer 70A and metalregions 70B. In the etching of metal seed layer 70A, alignment mark 62is also subject to damage. By adopting the grating patterns, however,alignment mark 62 can still be clearly viewed even with the damage.

FIG. 25 illustrates the formation of overlying structures, which aresimilar to what are shown in FIG. 16 . During the formation of RDLs 74and UBMs 78, a metal seed layer (not shown) may also be formed onalignment mark 62 and then etched. Alignment mark 62 thus suffers fromfurther damage. By adopting the grating patterns, however, alignmentmark 62 can still be clearly viewed even with the further damage. FIG.26 illustrates the demounting of wafer 100 from the respective carrier20 (FIG. 25 ), and the sawing of wafer 100 into packages 86. The sawingpasses through scribe lines 84, and some or all of alignment mark 62 areremoved, as discussed referring to FIG. 17 . FIG. 27 illustrates theresulting package 220.

In above-illustrated examples of the embodiments, some examples ofprocesses and features are discussed in accordance with some embodimentsof the present disclosure. Other features and processes may also beincluded. For example, testing structures may be included to aid in theverification testing of the 3D packaging or 3DIC devices. The testingstructures may include, for example, test pads formed in aredistribution layer or on a substrate that allows the testing of the 3Dpackaging or 3DIC, the use of probes and/or probe cards, and the like.The verification testing may be performed on intermediate structures aswell as the final structure. Additionally, the structures and methodsdisclosed herein may be used in conjunction with testing methodologiesthat incorporate intermediate verification of known good dies toincrease the yield and decrease costs.

The embodiments of the present disclosure have some advantageousfeatures. By adopting grating patterns and form elongate and narrowerstrips for alignment marks, it is easy to distinguish the alignmentmarks. The production tool used for alignment may provide scores for thealignment marks that are viewed in the manufacturing of the packages.The scores range between 0 and 100, with the score of 0 meaning noalignment mark is found, and the score of 100 meaning a perfectalignment mark image. The alignment mark images with the scores over 70are acceptable. The experimental results revealed that when a bulkalignment mark as shown in 29C-1 has a score of 42 or 50, which meansnot acceptable, the alignment mark shown in FIGS. 29A-1 and 29B-1 stillhave the scores higher than about 95 when all other conditions are thesame. Also, experimental results revealed that even if there are aplurality of dielectric layers over the grating alignment mark, thescores can still be maintained over 90.

In accordance with some embodiments of the present disclosure, a methodof forming a semiconductor device includes encapsulating a device die inan encapsulating material, forming a first dielectric layer over thedevice die and the encapsulating material, forming first redistributionlines extending into the first dielectric layer to electrically coupleto the device die, forming an alignment mark over the first dielectriclayer, wherein the alignment mark includes a plurality of elongatedstrips, forming a second dielectric layer over the first redistributionlines and the alignment mark, and forming second redistribution linesextending into the second dielectric layer to electrically couple to thefirst redistribution lines. The second redistribution lines are formedusing the alignment mark for alignment. In an embodiment, the formingthe second redistribution lines comprises forming via openings in thesecond dielectric layer, wherein portions of the first redistributionlines are exposed to the via openings, and the via openings are formedusing the alignment mark for alignment. In an embodiment, the pluralityof elongated strips in the alignment mark are parallel to each other,and are physically separated from each other. In an embodiment, theplurality of elongated strips in the alignment mark are interconnectedto form a ring. In an embodiment, the plurality of elongated strips inthe alignment mark each has a length/width ratio greater than about 5.In an embodiment, the plurality of elongated strips in the alignmentmark each has a width close to a minimum width allowed by a formationprocess of the alignment mark. In an embodiment, the firstredistribution lines and the alignment mark are formed in a commonformation process. In an embodiment, the alignment mark is formed in ascribe line, and the method further comprises cutting through the scribeline and the alignment mark.

In accordance with some embodiments of the present disclosure, a methodof forming a semiconductor device includes forming a through-via over acarrier; encapsulating a device die and the through-via in anencapsulating material; forming a first dielectric layer over andcontacting the device die, the through-via, and the encapsulatingmaterial; forming first via openings in the first dielectric layer toreveal the through-via and conductive features of the device die;plating first redistribution lines and an alignment mark, wherein thefirst redistribution lines comprise first via portions extending intothe first via openings, and the alignment mark comprises a firstplurality of elongated strips parallel to each other; forming a seconddielectric layer over the first dielectric layer, with the alignmentmark and first trace portions of the first redistribution lines embeddedin the second dielectric layer; forming second via openings in thesecond dielectric layer to reveal the first redistribution lines,wherein the second via openings are formed using the alignment mark foralignment; and plating second redistribution lines comprising second viaportions extending into the second via openings, and second traceportions over the second dielectric layer. In an embodiment, after theforming the second via openings, the alignment mark is covered by thesecond dielectric layer. In an embodiment, the method further includesforming a third dielectric layer over the second dielectric layer andthe second redistribution lines; forming third via openings in the thirddielectric layer to reveal the second redistribution lines, wherein thethird via openings are formed using the alignment mark for alignment,and after the forming the third via openings, the alignment mark isoverlapped by a portion of the third dielectric layer; and plating thirdredistribution lines comprising third via portions extending into thethird via openings, and third trace portions over the third dielectriclayer. In an embodiment, after the forming the second via openings, thealignment mark is revealed again, and in the plating the secondredistribution lines, a seed layer used for plating the secondredistribution lines is formed contacting the alignment mark, and themethod further comprises etching portions of the seed layer in contactwith the alignment mark. In an embodiment, the method further includesforming a third dielectric layer over the second dielectric layer andthe second redistribution lines; forming third via openings in the thirddielectric layer to reveal the second redistribution lines, wherein thethird via openings are formed using the alignment mark for alignment,and in the forming the third via openings, a portion of the thirddielectric layer directly over the alignment mark is removed, and a topsurface of the second dielectric layer is exposed; and plating thirdredistribution lines comprises third via portions extending into thethird via openings, and third trace portions over the third dielectriclayer In an embodiment, the alignment mark further comprises a secondplurality of elongated strips parallel to each other, and the firstplurality of elongated strips and the second plurality of elongatedstrips are connected to form a ring. In an embodiment, the firstplurality of elongated strips are separated from each other and have asubstantially uniform width. In an embodiment, the first plurality ofelongated strips are separated from each other and have a substantiallyuniform pitch.

In accordance with some embodiments of the present disclosure, a methodof forming a semiconductor device includes plating an alignment markover a first dielectric layer, wherein the alignment mark comprises aplurality of elongated strips parallel to each other, and the pluralityof elongated strips have a substantially uniform pitch and asubstantially uniform width; forming a plurality of redistribution linesover the first dielectric layer, wherein the plurality of redistributionlines are formed using the alignment mark for alignment; and sawingthrough the first dielectric layer and the alignment mark. In anembodiment, the forming the alignment mark comprises forming a seedlayer over the first dielectric layer; forming a patterned mask over theseed layer, with portions of the seed layer exposed through thepatterned mask; and plating the plurality of elongated strips of thealignment mark in the patterned mask. In an embodiment, the alignmentmark has a void, with some middle portions of the plurality of elongatedstrips being removed. In an embodiment, the plurality of elongatedstrips comprise a first plurality of elongated strips having a firstlength, and a second plurality of elongated strips having a secondlength greater than the first length.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: encapsulating a device diein an encapsulating material; forming a first dielectric layer over thedevice die and the encapsulating material; forming first redistributionlines extending into the first dielectric layer to electrically coupleto the device die; forming an alignment mark over the first dielectriclayer, wherein the alignment mark comprises a plurality of elongatedstrips; forming a second dielectric layer over the first redistributionlines and the alignment mark; removing a portion of the seconddielectric layer to expose the alignment mark; depositing a seed layer,wherein the seed layer comprises a first portion extending into thesecond dielectric layer to contact one of the first redistributionlines, and a second portion contacting the alignment mark; and formingsecond redistribution lines using the alignment mark for alignment. 2.The method of claim 1, wherein each of the plurality of elongated stripsin the alignment mark has a width and a length greater than the width,wherein the width is close to a minimum width allowed by a formationprocess of the alignment mark.
 3. The method of claim 1, wherein theportion of the second dielectric layer is removed before the seed layeris deposited.
 4. The method of claim 1 further comprising: forming athird dielectric layer over the second dielectric layer; forming thirdredistribution lines extending into the third dielectric layer toelectrically connected to the second redistribution lines; and after thethird redistribution lines are formed, removing a portion of the thirddielectric layer to reveal the alignment mark.
 5. The method of claim 1,wherein the plurality of elongated strips are connected to each other toform a ring.
 6. The method of claim 1 further comprising sawing thefirst dielectric layer and the second dielectric layer to form a packagecomprising the device die therein, wherein at a time the sawing isperformed, the alignment mark is exposed.
 7. The method of claim 6,wherein a portion of the alignment mark is removed by the sawing.
 8. Themethod of claim 6, wherein after the sawing, a part of the alignmentmark remains in the package.
 9. The method of claim 1 further comprisingbonding a package electrically connected to the second redistributionlines, wherein at a time the bonding is performed, the alignment mark isexposed.
 10. The method of claim 9 further comprising dispensing anunderfill, wherein the underfill physically contacts the alignment mark.11. The method of claim 1, wherein the plurality of elongated strips ofthe alignment mark are parallel to each other, with a void in theplurality of elongated strips.
 12. The method of claim 1, wherein theplurality of elongated strips of the alignment mark are physicallyconnected to form a full ring.
 13. A method comprising: forming athrough-via over a carrier; encapsulating a device die and thethrough-via in an encapsulant; forming first redistribution lines and analignment mark, wherein the first redistribution lines are electricallycoupled to the through-via and the device die, and the alignment markcomprises a plurality of elongated strips; forming a dielectric layerover the first redistribution lines, wherein the alignment mark andportions of the first redistribution lines are embedded in thedielectric layer; patterning the dielectric layer to expose thealignment mark and the first redistribution lines; and forming secondredistribution lines extending into the dielectric layer andelectrically coupling to the first redistribution lines, wherein thesecond redistribution lines are formed using the alignment mark foralignment.
 14. The method of claim 13, wherein after the patterning thedielectric layer, an entirety of the alignment mark is exposed.
 15. Themethod of claim 13, wherein the first redistribution lines and thealignment mark are formed in a common formation process.
 16. The methodof claim 13, wherein the alignment mark is formed in a scribe line, andthe method further comprises cutting through the scribe line, with aportion of the alignment mark being cut.
 17. The method of claim 13further comprising dispensing an underfill, wherein the underfill is inphysical contact with the alignment mark.
 18. A method comprising:plating an alignment mark over a first dielectric layer, wherein thealignment mark comprises a plurality of elongated strips; forming aplurality of redistribution lines over the first dielectric layer,wherein the plurality of redistribution lines are formed using thealignment mark for alignment; and dispensing an underfill, wherein theunderfill is in physical contact with top surfaces and sidewalls of thealignment mark.
 19. The method of claim 18, wherein the plurality ofelongated strips are parallel to each other, with at least two of theplurality of elongated strips having different lengths, and theplurality of elongated strips have a substantially uniform width, andwherein the alignment mark has a void surrounded by the plurality ofelongated strips, with some middle portions of some of the plurality ofelongated strips being removed.
 20. The method of claim 18 furthercomprising: forming a plurality of dielectric layers over the pluralityof redistribution lines; and performing a plurality of patterningprocesses, each patterning one of the plurality of dielectric layers toreveal the alignment mark.